An automatic machine such as a nut runner is employed for engagement of a bolt or nut with a corresponding nut or bolt. If there is a misalignment between the axes of the bolt and the nut, seizing will take place between the bolt and nut during their tightening. In order to eliminate this disadvantage, it has been proposed heretofore that a tapered guide is formed at the tip of a bolt or the edge of the threaded bore of a nut on the side of its seating face to pilot the bolt into the threaded bore of a nut or the threaded bore of the nut onto a bolt. This arrangement does not provide any piloting effect in the event that a misalignment between the bolt and the nut is significant.
An object of the invention is to provide a self-aligning bolt and a self-aligning nut capable of being tightened while effectively performing self-alignment with a conventional nut or bolt even if there is a significant misalignment between the bolt and the nut.